Effects of pulsed flows on nuisance periphyton growths in rivers: A mesocosm study

Abstract

Rivers regulated by dams are typically characterized by altered biotic communities and habitat structure in downstream reaches. In the Jackson River (Alleghany Co., VA), a relatively constant flow regime below Gathright Dam and anthropogenic nutrient loading have apparently contributed to nuisance levels of periphyton (>300 mg chlorophyll am-2). These nuisance growths cause low dissolved oxygen concentrations in the water column and altered benthic habitats in the Jackson River. The use of periodic pulsed flows has been suggested as a restoration practice that could potentially reduce periphyton biomass. We investigated the effects of increased flow on periphyton chlorophyll a (chl a), ash-free dry mass (AFDM), % organic matter (%OM) using streamside channels in which periphyton-colonized tiles were subjected to near-bed velocities ranging from 20 (control) to 240 cms -1. Analysis of variance (ANOVA) and regression were used to examine periphyton responses to velocity treatments. There was a significant decrease in chl a and AFDM, and significant increase in %OM in velocity treatments of 150, 180 and 240 cms -1 (p< 0.001), but not in lower velocity channels. Regression analyses showed a significant positive relationship with %OM (r2 1/4 0.88) and significant negative relationship with chl a (r2 1/40.77) and AFDM (r2 1/40.63). Algal taxa were dominated by Cladophora glomerata, Melosira varians and Pleurosira laevis. There was a significant positive relationship between treatment velocity and % C. glomerata (p1/40.007, r2 1/4 0.87) as diatoms were differentially removed with increasing treatment velocity. Our results demonstrate that pulsed flows can reduce periphyton standing crops in the Jackson River, but the discharge required to achieve this reduction would probably need to produce near-bed velocities >100cm s-1. Further study is needed to establish specific flow targets and evaluate the direct and indirect effects of pulsed flows on ecological conditions in the Jackson River. © 2009 John Wiley & Sons, Ltd.